It has been estimated that 70% of adults have had a significant episode of back pain or chronic back pain emanating from a region of the spinal column or backbone. Many people suffering chronic back pain or an injury requiring immediate intervention resort to surgical intervention to alleviate their pain.
The spinal column or backbone encloses the spinal cord and consists of 33 vertebrae superimposed upon one another in a series which provides a flexible supporting column for the trunk and head. The vertebrae cephalad (i.e., toward the head or superior) to the sacral vertebrae are separated by fibrocartilaginous intervertebral discs and are united by articular capsules and by ligaments. The uppermost seven vertebrae are referred to as the cervical vertebrae, and the next lower twelve vertebrae are referred to as the thoracic, or dorsal, vertebrae. The next lower succeeding five vertebrae below the thoracic vertebrae are referred to as the lumbar vertebrae and are designated L1-L5 in descending order. The next lower succeeding five vertebrae below the lumbar vertebrae are referred to as the sacral vertebrae and are numbered S1-S5 in descending order. The final four vertebrae below the sacral vertebrae are referred to as the coccygeal vertebrae. In adults, the five sacral vertebrae fuse to form a single bone referred to as the sacrum, and the four rudimentary coccyx vertebrae fuse to form another bone called the coccyx or commonly the “tail bone”. The number of vertebrae is sometimes increased by an additional vertebra in one region, and sometimes one may be absent in another region.
Typical lumbar, thoracic and cervical vertebrae consist of a ventral or vertebral body and a dorsal or neural arch. In the thoracic region, the ventral body bears two costal pits for reception of the head of a rib on each side. The arch which encloses the vertebral foramen is formed of two pedicles and two lamina. A pedicle is the bony process which projects backward or anteriorly from the body of a vertebra connecting with the lamina on each side. The pedicle forms the root of the vertebral arch. The vertebral arch bears seven processes: a dorsal spinous process, two lateral transverse processes, and four articular processes (two superior and two inferior). A deep concavity, inferior vertebral notch, on the inferior border of the arch provides a passageway or spinal canal for the delicate spinal cord and nerves. The successive vertebral foramina surround the spinal cord. Articulating processes of the vertebrae extend posteriorly of the spinal canal.
The bodies of successive lumbar, thoracic and cervical vertebrae articulate with one another and are separated by the intervertebral spinal discs. Each spinal disc comprises a fibrous cartilage shell enclosing a central mass, the “nucleus pulposus” (or “nucleus” herein) that provides for cushioning and dampening of compressive forces to the spinal column. The shell enclosing the nucleus comprises cartilaginous endplates adhered to the opposed cortical bone endplates of the cephalad and caudal vertebral bodies and the “annulus fibrosis” (or “annulus” herein) comprising an annular fibrosis layer of collagen fibers running circumferentially around the nucleus pulposus and connecting the cartilaginous endplates. The nucleus contains hydrophilic (water attracting) micropolysacharides and fibrous strands. The nucleus is relatively inelastic, but the annulus can bulge outward slightly to accommodate loads axially applied to the spinal motion segment.
The intervertebral discs are anterior to the spinal canal and located between the opposed end faces or endplates of a cephalad and a caudal vertebral body. The inferior articular processes articulate with the superior articular processes of the next succeeding vertebra in the caudal (i.e., toward the feet or inferior) direction. Several ligaments (supraspinous, interspinous, anterior and posterior longitudinal, and the ligamenta flava) hold the vertebrae in position yet permit a limited degree of movement. The assembly of two vertebral bodies, the interposed, intervertebral, spinal disc and the attached ligaments, muscles and facet joints is referred to as a “spinal motion segment”.
The relatively large vertebral bodies located in the anterior portion of the spine and the intervertebral discs provide the majority of the weight bearing support of the vertebral column. Each vertebral body has relatively strong, cortical bone layer comprising the exposed outside surface of the body, including the endplates, and weak, cancellous bone comprising the center of the vertebral body.
A number of spinal disorders are caused by traumatic spinal injuries, disease processes, aging processes, and congenital abnormalities that cause pain, reduce the flexibility of the spine, decrease the load bearing capability of the spine, shorten the length of the spine, and/or distort the normal curvature of the spine. These spinal disorders and various treatments that have been clinically used or proposed are first described as follows.
With aging, the nucleus becomes less fluid and more viscous and sometimes even dehydrates and contracts (sometimes referred to as “isolated disc resorption”) causing severe pain in many instances. In addition, the annulus tends to thicken, desiccate, and become more rigid, lessening its ability to elastically deform under load and making it susceptible to fracturing or fissuring.
One form of degeneration of the disc occurs when the annulus fissures or is rent. The fissure may or may not be accompanied by extrusion of nucleus material into and beyond the annulus. The fissure itself may be the sole morphological change, above and beyond generalized degenerative changes in the connective tissue of the disc, and disc fissures can nevertheless be painful and debilitating. Biochemicals contained within the nucleus are alleged to escape through the fissure and irritate nearby structures.
A fissure also may be associated with a herniation or rupture of the annulus causing the nucleus to bulge outward or extrude out through the fissure and impinge upon the spinal column or nerves (a “ruptured” or “slipped” disc). With a contained disc herniation, the nucleus may work its way partly through the annulus but is still contained within the annulus or beneath the posterior longitudinal ligament, and there are no free nucleus fragments in the spinal canal. Nevertheless, even a contained disc herniation is problematic because the outward protrusion can press on the spinal cord or on spinal nerves causing sciatica.
Another disc problem occurs when the disc bulges outward circumferentially in all directions and not just in one location. This occurs when over time, the disc weakens, bulges outward and takes on a “roll” shape. Mechanical stiffness of the joint is reduced and the spinal motion segment may become unstable shortening the spinal cord segment. As the disc “roll” extends beyond the normal circumference, the disc height may be compromised, and foramina with nerve roots are compressed causing pain. In addition, osteophytes may form on the outer surface of the disc roll and further encroach on the spinal canal and foramina through which nerves pass. The cephalad vertebra may eventually settle on top of the caudal vertebra. This condition is called “lumbar spondylosis”.
In addition, various types of spinal column displacement disorders are known in one or more spinal motion segment that are hereditary or are caused by degenerative disease processes or trauma. Such spinal displacement disorders include scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine), spondylolisthesis (forward displacement of one vertebra over another, usually in the lumbar or cervical spine). At times the displacement disorder is accompanied by or caused by a fracture or partial collapse of one or more vertebrae or degeneration of a disc. Patients who suffer from such conditions can experience moderate to severe distortion of the thoracic skeletal structure, diminished ability to bear loads, loss of mobility, extreme and debilitating pain, and oftentimes suffer neurologic deficit in nerve function.
Approximately 95% of spinal surgery involves the lower lumbar vertebrae designated as the fourth lumbar vertebra (“L4”), the fifth lumbar vertebra (“L5”), and the first sacral vertebra (“S1”). Persistent low back pain is attributed primarily to degeneration of the disc connecting L5 and S1. Traditional, conservative methods of treatment include bed rest, pain and muscle relaxant medication, physical therapy or steroid injection. Upon failure of conservative therapy spinal pain (assumed to be due to instability) has traditionally been treated by spinal fusion, with or without instrumentation, which causes the vertebrae above and below the disc to grow solidly together and form a single, solid piece of bone.
Highly invasive, open surgical procedures have been developed and used to perform a “complete discectomy” to surgically remove the disc, and the vertebral bodies are then fused together. The removal of the disc involves removing the nucleus, cutting away the cartilaginous endplates adhered to the opposed cortical bone endplates of the cephalad and caudal vertebral bodies, and removing at least a portion of the annulus. Fusion of the vertebral bodies involves preparation of the exposed endplate surfaces by decortication (scraping the endplate cortical bone) and the deposition of additional bone into disc space between the prepared endplate surfaces. The complete discectomy and fusion may be performed through a posterior surgical route (from the back side of the patient) or an anterior surgical route (from the front side of the patient). The removed vertebral bone may be just the hard cortical bone or may include soft cancellous soft bone in the interior of the vertebral bodies. Controversy exists regarding the preferred method of performing these fusions for various conditions of the spine. Sometimes, non-biological materials are used to augment and support the bone grail (fixation systems). Sometimes, the fixation is performed from the posterior route (posterior fixation), or from the anterior route (anterior fixation), or even both sides (anterior-posterior fixations or circumferential fusion).
Current treatment methods other than spinal fusion for symptomatic disc rolls and herniated discs include “laminectomy” which involves the lateral surgical exposure of the annulus and surgical excision of the symptomatic portion of the herniated disc followed by a relatively lengthy recuperation period.
Various other surgical treatments that attempt to preserve the intervertebral spinal disc and to simply relieve pain include a “nucleotomy” or “disc decompression” to remove some or most of the interior nucleus thereby decompressing and decreasing outward pressure on the annulus. In less invasive microsurgical procedures known as “microlumbar discectomy” and “automated percutaneous lumbar discectomy”, the nucleus is removed by suction through a needle laterally extended through the annulus. Although these procedures are less invasive than open surgery, they nevertheless suffer the possibility of injury to the nerve root and dural sac, perineural scar formation, reherniation of the site of the surgery, and instability due to excess bone removal. Moreover, they involve the perforation of the annulus.
Another method of treatment is known as “chemonucleolysis”, which is carried out by injection of the enzyme chymopapain into the nucleus through the annulus. This procedure has many complications including severe pain and spasm, which may last up to several weeks following injection. Sensitivity reactions and anaphylactic shock occur in limited but significant numbers of patients.
Although damaged discs and vertebral bodies can be identified with sophisticated diagnostic imaging, the surgical procedures are so extensive that clinical outcomes are not consistently satisfactory. Furthermore, patients undergoing such fusion surgery experience significant complications and uncomfortable, prolonged convalescence. Surgical complications include disc space infection, nerve root injury, hematoma formation, and instability of adjacent vertebrae.
Many surgical techniques, instruments and spinal disc implants have been described in the medical literature and in patents that are directed to providing less invasive, percutaneous, lateral access to a degenerated intervertebral spinal disc. Then, instruments are introduced through lateral disc openings made through the annulus for performing a discectomy and implanting bone growth materials or biomaterials or spinal disc implants inside the annulus. Or, one or more laterally extending space or hole is bored through the disc to receive one or more laterally inserted spinal disc implant or bone growth material to promote fusion or to receive a pre-formed, artificial, functional disc replacement implant as typified by U.S. Pat. No. 5,700,291.
Percutaneous lateral procedures and instruments for performing such discectomies are disclosed in U.S. Pat. Nos. Re. 33,258, 4,573,448, 5,015,255, 5,313,962, 5,383,884, 5,702,454, 5,762,629, 5,976,146, 6,095,149, and 6,127,597 and in PCT publication WO 99/47055, for example. A laparascopic technique and apparatus for traversing the retroperitoneal space from an abdominal skin incision to an anterior surface of the disc annulus and performing a discoscopy is disclosed in the '962 patent, for example. Percutaneous surgical disc procedures and apparatus that accesses the disc in a posterolateral approach from a skin incision in the patient's back are described in the '629 and '448 patents, for example.
The nucleus is fragmented by various mechanical cutting heads as disclosed, for example in the '258, '962, '884, and '597 patents, for example. Or, thermal or laser energy is applied to desiccate the nucleus and to stiffen the annulus as described in the '149 patent, for example. Or, the nucleus and portions of the cephalad and caudal vertebral bodies are mechanically cut away to enlarge the disc space as described in the PCT '055 publication and in the '255 patent, for example. Irrigation fluid is introduced into the disc space or cavity and the fragments or desiccation by-products of the nucleus and any bone and annulus fragments are aspirated from the disc space or cavity. The irrigation and aspiration is effected through an access cannula positioned against the opening through the annulus of the herniated disc as disclosed in the '629 patent, for example, or through a lumen of the discectomy instrument, as disclosed in the '258 patent, for example. A measure of safety and accuracy is added to these operative procedures by the artiroscopic visualization of the annulus and other important structures which lie in the path of the instruments, such as the spinal nerve.
The above-described procedures involve invasive surgery that laterally exposes the anterior or posterior (or both) portions of the vertebrae and intervertebral spinal disc. Extensive muscular stripping and bone preparation can be necessary. As a result, the spinal column can be further weakened and/or result in surgery induced pain syndromes. Thus, presently used or proposed surgical fixation and fusion techniques involving the lower lumbar vertebrae suffer from numerous disadvantages.
Methods and apparatus for accessing the discs and vertebrae by lateral surgical approaches that purportedly reduce muscular stripping (and that are similar to those disclosed in the above-referenced '629 and '888 patents) are described in U.S. Pat. No. 5,976,146. The intervening muscle groups or other tissues are spread apart by a cavity forming and securing tool set disclosed in the '146 patent to enable endoscope aided, lateral access to damaged vertebrae and discs and to perform corrective surgical procedures. However, it is preferable to avoid the lateral exposure to correct less severe spondylolisthesis and other spinal injuries or defects affecting the lumbar and sacral vertebrae and discs.
A less intrusive posterior approach for treating spondylolisthesis is disclosed in U.S. Pat. No. 6,086,589, wherein a straight bore is formed through the sacrum from the exposed posterior sacral surface and in a slightly cephalad direction into the L5 vertebral body, preferably after realigning the vertebrae. A straight, hollow, threaded shaft with side wall holes restricted to the end portions thereof and bone growth material are inserted into the bore. A discectomy of the disc between L5 and S1 is preferably performed in an unexplained manner, and bone ingrowth material is also preferably inserted into the space between the cephalad and caudal vertebral bodies. Only a limited access to and alignment of S1 and L5 can be achieved by this approach because the distal ends of the straight bore and shaft approach and threaten to perforate the anterior surface of the L5 vertebral body. This approach is essentially a posteriolateral approach that is intended to fuse S1 and L5 and cannot access more cephalad vertebral bodies or intervertebral spinal discs.
In many of these procedures, a laterally extending space is prepared by removal of the disc to receive one or more disc implant, and insertion of a bone growth material, e.g. autologous bone, or a pre-formed, artificial, functional disc replacement implant. A number of disc shaped, functional disc replacement implants and methods of insertion have been proposed as disclosed, for example, in U.S. Pat. Nos. 5,258,031 and 6,019,792, for example. Other disc shaped or vertebral body replacement implants that are designed to encourage bone growth and effect fusion are shown in U.S. Pat. Nos. 5,514,180 and 5,888,223, for example. These devices and techniques are intended to overcome the disadvantages of purely surgical techniques to mechanically immobilize areas of the spine assisting in the eventual fusion of the treated adjacent vertebrae, and to maintain the length of the treated spinal motion segment to avoid shortening spinal cord and nerve segments. However, they require relatively large lateral exposure of the disc or vertebral body to excise the disc or vertebral body, shape the adjoining caudal and cephalad vertebral bodies and effect the implantation and fixation thereto. Thus, disadvantages to the present implants and surgical implantation techniques remain concerning the implantation procedures and involving post-surgical failure necessitating re-operation.
A further type of disc implant that has been clinically employed for spinal fusion comprises a hollow, cylindrical, titanium cage that is externally threaded and is screwed laterally into place in a lateral bore formed through the disc between two adjacent vertebrae. Typically, the lateral bore involves complete discectomy of the damaged disc and removal of portions of the cortical bone of the adjoining vertebral bodies to prepare a laterally and axially extending space to receive one or more disc implant. Bone grafts from cadavers or the pelvis or substances that promote bone growth are then packed into the hollow center of the cage to encourage bone growth (or ingrowth) through the cage pores to achieve fusion of the two adjacent vertebrae. Two such cage implants and the surgical tools employed to place them are disclosed in U.S. Pat. Nos. 5,505,732 and 5,700,291, for example. The cage implants and the associated surgical tools and approaches require precise drilling of a relatively large hole for each such cage laterally between two adjacent vertebral bodies and then threading a cage into each prepared hole. The exposed ends of the cage or side by side installed cages can irritate nerves causing pain to emerge again.
These approaches involve a virtually complete discectomy of the disc achieved by instruments introduced laterally through the patient's body to the disc site and manipulated to cut away or drill lateral holes through the disc and adjoining cortical bone. The large laterally drilled hole or holes can compromise the integrity of the vertebral bodies, and the spinal cord can be injured if they are drilled too posteriorly. The endplates of the vertebral bodies, which comprise very hard cortical bone and help to give the vertebral bodies needed strength, are usually weakened or destroyed during the drilling. The cylindrical cage or cages are now harder than the remaining bone of the vertebral bodies, and the vertebral bodies tend to collapse or “telescope” together. The telescoping causes the length of the vertebral column to shorten and can cause damage to the spinal cord and nerves that pass between the two adjacent vertebrae.
Therefore, it is often necessary to also mechanically stabilize the vertebrae on either side of the spinal disc that is augmented or removed so that fusion of the vertebral bodies can occur successfully without telescoping of the vertebral bodies or movement of the disc implants out of the prepared site. One technique for spinal fixation includes the immobilization of the spine by the use of spine rods of many different configurations that run generally parallel to the spine. Typically, the posterior surface of the spine is isolated and bone screws are first fastened to the pedicles of the appropriate vertebrae or to the sacrum and act as anchor points for the spine rods. The bone screws are generally placed two per vertebra, one at each pedicle on either side of the spinous process. Clamp assemblies join the spine rods to the screws. The spine rods are generally bent to achieve the desired curvature of the spinal column. Wires may also be employed to stabilize rods to vertebrae. These techniques are described further in U.S. Pat. No. 5,415,661, for example.
These types of rod systems can be effective, but require a posterior approach and implanting screws into or clamps to each vertebra over the area to be treated. To stabilize the implanted system sufficiently, one vertebra above and one vertebra below the area to be treated are often used for implanting pedicle screws. Since the pedicles of vertebrae above the second lumbar vertebra (L2) are very small, only small bone screws can be used which sometimes do not give the needed support to stabilize the spine. These rods and screws and clamps or wires are surgically fixed to the spine from a posterior approach, and the procedure is difficult. A large bending moment is applied to such rod assemblies, and because the rods are located outside the spinal column, they depend on the holding power of the associated components which can pull out of or away from the vertebral bone.
In a further approach disclosed in U.S. Pat. Nos. 4,553,273 and 4,636,217, both described in U.S. Pat. No. 5,735,899, two of three vertebrae are joined by surgically obtaining access to the interior of the upper and lower vertebral bodies through excision of the middle vertebral body. In the '899 patent, these approaches are referred to as “intraosseous” approaches, although they are more properly referred to as “interosseous” approaches by virtue of the removal of the middle vertebral body. The removal is necessary to enable a lateral insertion of the implant into the space it occupied so that the opposite ends of the implant can be driven upward and downward into the upper and lower vertebral bodies. These approaches are criticized as failing to provide adequate medial-lateral and rotational support in the '899 patent. In the '899 patent, an anterior approach is made, slots are created in the upper and lower vertebrae, and rod ends are fitted into the slots and attached to the remaining vertebral bodies of the upper and lower vertebrae by laterally extending screws. These approaches involve considerable damage to ligaments and tissue in the anterior access to the vertebral bones.
The use of radiopaque metal cages or other metal implants also makes it difficult to image the disc space with radiographic imaging equipment to assess the degree of fusion achieved by bone growth between the vertebral bodies separated by the cages. Laterally insertable, rigid carbon fiber and more flexible polymeric disc implants are under study as replacements for metal implants.
Alternatively, the use of a deflated porous fabric bag that is laterally inserted into a prepared cavity and inflated with bone growth encouraging material is disclosed in U.S. Pat. No. 5,549,679. The prepared cavity is substantially ovaloid and includes the removed disc and a portion of the adjoining vertebral bodies. The filling of the bag under pressure tends to distract, i.e., to separate, the adjoining vertebral bodies to the physiologic separation that would be provided by the undamaged disc. The porous bag opening is closed in a number of ways to retain the material it is filled with. This porous bag is distinguished from several other artificial disc designs described in the '679 patent, including an artificial disc with an elastomeric core (U.S. Pat. No. 5,071,437) or filled with hydrogel beads (U.S. Pat. No. 5,192,326).
In a further disc augmentation approach described in U.S. Pat. No. 5,888,220, the disc is accessed laterally through the patient's body, the annulus is perforated unless it is already rent, and a partial discectomy is performed to remove most or all of the nucleus to create a space within the annulus. Then, a mass of curable biomaterials is injected into the prepared space and the material is cured in situ. In one variation, a deflated balloon is inserted into the prepared space, and the mass of curable biomaterials is injected into the prepared space and the material is cured in situ, leaving the filled balloon and solidified biomaterial in place.
A compilation of many of the above described surgical techniques and spinal implants and others that have been used clinically is set forth in certain chapters of the book entitled Lumbosacral and Spinopelvic Fixation, edited by Joseph Y. Margolies et al. (Lippincott-Raven Publishers, Philadelphia, 1996). Attention is directed particularly to Chapters 1, 2, 17, 18, 38, 42 and 44.
In “Lumbopelvic Fusion” (Chapter 38, by Prof. Rene P. Louis, MD) techniques for repairing a spondylolisthesis, in this case, a severe displacement of L5 with respect to S1 and the intervening disc, are described and depicted. An anterior lateral exposure of L5 and S1 is made, a discectomy is performed, and the orientation of L5 to S1 is mechanically corrected using a reduction tool, if the displacement is severe. A fibula graft or metal Judet screw is inserted as a dowel through a bore formed extending caudally through L5 and into S1. When the screw is used, bone growth material, e.g., bone harvested from the patient, is inserted into the bore alongside the screw, and the disc space is filled with bone sutured to the screw to keep it in place between the vertebral surfaces to act as a spacer implant occupying the extracted disc between L5 and S1. External bridge plates or rods are also optionally installed. The posterolateral or anterior lateral approach is necessitated to correct the severe spondylolisthesis displacement using the reduction tool and results in tissue injury. Because of this approach and need, the caudal bore and inserted the Judet screw can only traverse L5 and S1.
A similar anterior approach for treating spondylolisthesis is disclosed in U.S. Pat. No. 6,056,749. In this approach, a bore hole is formed in a cephalad vertebral body and extends through the intervening disc into a caudal vertebral body, the disc is removed, a disk cage is inserted laterally into the disc space, and an elongated, hollow threaded shaft is inserted into the bore and through a hole in the disc cage. The disk cage takes the place of the harvested bone disc inserts and its interlocking intersection with the shaft takes the place of the sutures employed to tie the harvested bone disc inserts to the screw in the technique described in the above-referenced Chapter 38 publication.
Turning to a further spinal disorder, the vertebral bodies can thin and weaken with the development and progression of osteoporosis and certain eating disorders to the point that one or more vertebral body compression fractures occur as described in U.S. Pat. Nos. 4,969,888, 5,972,015 and 6,066,154. Vertebral compression fractures of healthy vertebral bodies can also occur due to injury. In severe cases, the vertebral body tends to collapse, shortening the vertebral body and the spine and inducing an aberrant localized spinal curvature. As noted in the '888 patent, osteoporotic vertebral body compression fractures are currently treated with bed rest, analgesics, and intravenous hydration during the first week after onset of the problem. These steps are followed by the prescription of a soft or firm spinal corset, depending upon the physician's preference. In most cases, the corset is not worn because the patient suffers much discomfort and oftentimes greater discomfort than that due to the fracture of the vertebral body. The fracture pain lasts from two to eight months. In many cases, patients with osteoporotic vertebral body collapse fractures require about one week in an acute care hospital and two to three weeks in an extended care facility until they are able to move about independently and with only moderate pain. Current treatment does not substantially alter the conditions of the vertebral body.
The '888 patent describes a “balloon-assisted vertebroplasty” method of restoring the vertical height of a collapsed, compression fractured vertebral bone through a posterolateral approach from an entry point on the skin determined radiologically and is located approximately 10 cm from the midline and just inferior to a rib if present at that level. A guide pin is extended from the incision to the vertebral body and through the cortical bone and a predetermined distance into the cancellous bone. A cannula is inserted over the guide pin and its distal end is attached to the exterior cortical bone of the vertebral body. A drill is extended through the cannula and used to drill a hole into the cancellous bone to enlarge the cavity to be treated. A deflated, expandable balloon is inserted through the cannula and inflated inside the vertebral body into a disc or checker shape. The expansion of the balloon compacts the cancellous bone against the inner surface of the outer cortical wall of the vertebral body thereby further enlarging the cavity and, it is asserted, filling the fractures in the cortical bone. The balloon expansion may also restore the height of the vertebral body to some extent. The balloon is then deflated and removed, and the cavity is irrigated with saline. The cavity is simultaneously aspirated and filled with a flowable synthetic bone material or methyl methacrylate cement that is allowed to set to a hardened condition through the cannula. It is asserted that the compacted cortical bone or bone marrow will substantially prevent flow through the fracture.
The '015 and '154 patents disclose generally the same procedure steps but employ improved, irregularly shaped, balloons that approximate the inner shape of the vertebral bodies they are inflated within in order to maximally compress cancellous bone. The balloons are made of inelastic material and are kept in their defined configurations when inflated by various shape restraints. This procedure is also referred to as a “Kyphoplasty”, by Kyphon, Inc., the assignee of the '015 and '154 patents.
There are other therapeutic treatments for encouraging bone growth within a vertebral body or to fuse vertebral bodies together with or without a pre-formed spinal disc replacement implant that involve injection of bone growth materials into the disc or vertebral body or the application of electrical energy to stimulate bone growth. Several natural or artificial osteoconductive, osteoinductive, osteogenic or other fusion enhancing materials are disclosed in U.S. Pat. No. 6,123,705. A system and method for delivering electrical energy to a pre-formed spinal disc replacement implant to promote bone growth and fusion about the implant and between the opposed endplates of the cephalad and caudal vertebral bodies are disclosed in U.S. Pat. No. 6,120,502.
The above-described procedures involve invasive surgery that laterally exposes the anterior or posterior (or both) portions of the vertebrae and intervertebral disc. Extensive muscular stripping and bone preparation can be necessary. As a result, the spinal column can be further weakened and/or result in surgery induced pain syndromes. Thus, presently used or proposed surgical fixation and fusion techniques involving the lower lumbar vertebrae suffer from numerous disadvantages.
Methods and apparatus for accessing the discs and vertebrae by lateral surgical approaches that purportedly reduce muscular stripping (and that are similar to those disclosed in the above-referenced '629 and '888 patents) are described in U.S. Pat. No. 5,976,146. The intervening muscle groups or other tissues are spread apart by a cavity forming and securing tool set disclosed in the '146 patent to enable endoscope aided, lateral access to damaged vertebrae and discs and to perform corrective surgical procedures. However, it is preferable to avoid the lateral exposure to correct less severe spondylolisthesis and other spinal injuries or defects affecting the lumbar and sacral vertebrae and discs.
A less intrusive posterior approach for treating spondylolisthesis is disclosed in U.S. Pat. No. 6,086,589, wherein a straight bore is formed through the sacrum from the exposed posterior sacral surface and in a slightly cephalad direction into the L5 vertebral body, preferably after realigning the vertebrae. A straight, hollow, threaded shaft with side wall holes restricted to the end portions thereof and bone growth material are inserted into the bore. A discectomy of the disc between L5 and S1 is preferably performed and bone ingrowth material is also preferably inserted into the space between the cephalad and caudal vertebral bodies. Only a limited access to and alignment of S1 and L5 can be achieved by this approach because the distal ends of the straight bore and shaft approach and threaten to perforate the anterior surface of the L5 vertebral body. This approach is essentially a posteriolateral approach that is intended to fuse S1 and L5 and cannot access more cephalad vertebral bodies or intervertebral discs.
A wide variety of orthopedic implants have also been proposed or clinically employed to stabilize broken bones or secure artificial hip, knee and finger joints. Frequently, rods or joint supports are placed longitudinally within longitudinal bores made in elongated bones, e.g., the femur. A surgical method is disclosed in U.S. Pat. No. 5,514,137 for stabilizing a broken femur or other long bones using an elongated rod and resorbable cement. To accomplish a placement of a rod into any single bone, an end of a bone is exposed and a channel is drilled from the exposed end to the other end. Thereafter, a hollow rod is inserted, and resorbable cement is injected through the hollow rod, so as to provide fixation between the distal end of the rod and the cancellous tissue that surrounds the rod. A cement introducer device can also be used for the injection of cement. A brief reference is made in the '137 patent to the possibility of placing rods in or adjacent to the spine in the same manner, but no particular approach or devices are described.
Drilling tools are employed in many of the above described surgical procedures to bore straight holes into the vertebral bones. The boring of curved bores in other bones is described in U.S. Pat. Nos. 4,265,231, 4,541,423, and 5,002,546, for example. The '231 patent describes an elongated drill drive shaft enclosed within a pre-curved outer sheath that is employed to drill curved suture holding open ended bores into bones so that the suture passes through both open ends of the bore. The '423 patent describes an elongated flexible drill drive shaft enclosed within a malleable outer sheath that can be manually shaped into a curve before the bore is formed. The '546 patent describes a complex curve drilling tool employing a pivotal rocker arm and curved guide for a drill bit for drilling a fixed curve path through bone. All of these approaches dictate that the curved bore that is formed follow the predetermined and fixed curvature of the outer sheath or guide. The sheath or guide is advanced through the bore as the bore is made, making it not possible for the user to adjust the curvature of the bore to track physiologic features of the bone that it traverses.
All of the above-described patents and other patents referenced herein that access a single spinal disc or vertebra to perform the above-described therapies, do so from a lateral approach that involves weakening of the spinal fusion segment. There remains a need for methods and apparatus for performing therapeutic procedures in the spine in a minimally invasive, low trauma, manner.